A sensor for an intruder detection system is typically formed of a leaky (radiating) coaxial cable, to one end of which is connected a transmitter, typically operating at 40 MHz CW. The radiated field of the transmitted signal penetrates a parallel leaky receiving cable spaced typically 3-8 feet away, and is received by a receiver connected to one end of the receiving cable. When an intruder passes into the radiating field penetrating the received cable, it causes an amplitude and phase change in the field, which is detected in the receiver, thus determining that an intruding body is present. The cables can be either buried or located at or above ground level. Intruder detection systems of this type have been described in a paper by Dr. R. Keith Harman and John E. Siedlarz, given to the 1982 Carnahan Conference on Security Technology, at the University of Kentucky, May 12-14, 1982. While early papers suggest operation on or above ground, this has not proven to be feasible due to huge environmental effects for cables on the surface and mode cancellations for air mounted cables.
In the case of buried cables, changes in the dielectric constant of the burial medium, e.g. local wet, sandy, oily, etc. regions, significantly affect the sensitivity of the system, so that long sensors often have extreme high sensitivity regions adjacent certain portions of the sensor and poor sensitivity (null) regions adjacent other portions. This can cause generation of false alarms and points of undetectable intrusion. In addition, it is costly to dig two spaced trenches for burial of the cable; in case of a requirement for service, two trenches must be dug up.
Cables located at or above the ground level are visible, thus allowing potential intruders to note and possibly avoid their positions, but also exhibit regularly spaced peaks and valleys in sensitivity. Consequently above ground cable sensors are usually avoided wherever possible.
The present invention is directed to a leaky cable which can be used in a sensor or as an antenna, and to a sensor which is substantially insensitive to variations in dielectric constant and conductivity in the burial medium of a sensor. The sensor containing both transmitting and receiving elements can be manufactured as a single cable, and thus only a single trench need be dug for its burial. The same cable can be used at or above ground level with substantial reduction or elimination of the peaks and nulls exhibited by prior art above-ground sensors. Accordingly a sensor or radiating cable can be used above ground for the first time with predictability and confidence that peaks and nulls will not significantly affect sensor performance.